WHen I send emails to certain user via Outlook 2007 they don’t get it after a couple days I get a NDR #4.4.7, the users never get the mails and I am notified way to late that they hadn’t received emails.

However I do notice that when I attempt to send the email by OWA it goes without any issues.

My emails are hosted on a Win 2k3 server and Exchange 2k3 that is my domain controller. I host my own emails.

After sending the message did you check if it’s actually stuck in ESM in Queues? You’re not saying anything about where’s the NDR originating. Is it from your server or some remote mail server. Check email header.
yaro